High Voltage Direct Current (HVDC) transmission systems comprise an interesting alternative to alternating current ditto, and are under development. A difficulty when developing HVDC systems, and in particular when designing HVDC grids, is the provision of breakers that are able to break the high voltage direct current. Mechanical switches suffer from long response times, i.e. they are simply too slow to meet various requirements. Further, arcing may be another difficulty of such mechanical switches and has to be taken into consideration. Further, the time to clear a fault may be very long, which may be accounted for by over dimensioning components so that they are able to withstand fault currents and/or fault voltages for a prolonged duration. Over dimensioning of components in a power system however translates into increased costs and often also into larger footprint requirements.
Semiconductor-based switches are fast and could be used for HVDC applications. However, a large number of semiconductor devices would be required for the high voltages and currents, which would again give an expensive solution and which would typically require a large footprint.
The use of electron tubes, based on vacuum technology, has been discussed over the years as an alternative, and recently, cold cathode electron tubes able to withstand high voltages and currents have drawn attention. The use of electron tubes in high voltage direct current applications requires various considerations.